1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a system and a method for transmitting/receiving signals in a mobile communication system using a multiple input multiple output adaptive antenna array scheme.
2. Description of the Related Art
Packet service communication systems have been developed as next-generation mobile communication systems. Such packet service communication systems transmit burst packet data to a plurality of mobile stations and are adaptable to transmit mass storage data. Recently, various packet service communication systems are being developed in order to provide a high-speed packet service. The 3GPP (3rd Generation Partnership Project), which is a consortium established for providing the asynchronous telecommunication system standards, has suggested a high-speed downlink packet access (hereinafter, simply referred to as “HSDPA”) scheme for providing the high-speed packet service. In addition, the 3GPP2 (3rd Generation Partnership Project2), which is a consortium established for providing the synchronous telecommunication system standards, has suggested a 1× EV-DO/V (1× Evolution Data Only/Voice) scheme for providing the high-speed packet service. Both HSDPA and 1× EV-DO/V schemes suggest utilizing the high-speed packet service in order to easily transmit Internet services, such as web services. When providing such a high-speed packet service, a peak throughput as well as an average throughput must be optimized in order to easily transmit packet data and circuit data, such as voice services.
In particular, in order to allow a communication system using the HSDPA scheme (hereinafter, simply referred to as “HSDPA communication system”) to transmit the high-speed packet data, three schemes are newly provided for the HSDPA communication system. The three new schemes include an adaptive modulation and coding (hereinafter, simply referred to as “AMC”) scheme, a hybrid automatic retransmission request (hereinafter, simply referred to as “HARQ”) scheme, and a fast cell select (hereinafter, simply referred to as “FAC”) scheme. The HSDPA communication system improves a data transmission rate thereof by using the AMC scheme, the HARQ scheme and the FCS scheme. The HSDPA communication system has been described herein as an example, and a communication system using a 1× EV-DO/V scheme (hereinafter, simply referred to as “1× EV-DO/V communication system”) may be provided in order to improve the data transmission rate. In order to improve performance of the 1× EV-DO/V communication system, the data transmission rate thereof must be increased. Besides the above new schemes, such as the AMC scheme, the HARQ scheme and the FCS scheme, a multiple antenna scheme can be used in order to increase the data transmission rate while overcoming a limitation of an assigned bandwidth. Such a multiple antenna scheme utilizes a space domain in order to overcome the limitation of bandwidth resources in a frequency domain.
Hereinafter, the multiple antenna scheme will be described.
Firstly, a mobile communication system is constructed such that it communicates with a plurality of mobile stations through a base station. If the base station transmits high-speed data to the mobile stations, a fading phenomenon may occur due to the characteristics of a radio channel. In order to overcome the fading phenomenon, a transmission antenna diversity scheme, which is a multiple antenna scheme, has been suggested. According to the transmission antenna diversity scheme, signals are transmitted through at least two antennas, such that a transmission loss of the data caused by the fading phenomenon can be minimized, thereby increasing the data transmission rate.
In general, and different from a wired channel environment, a radio channel environment existing in a mobile communication system is subject to various parameters, such as a multipath interference, shadowing, wave attenuation, noise and interference. This being the case, a radio channel may receive a signal which has been distorted from the original transmission signal. The fading phenomenon caused by the multipath interference is closely related to the mobility of a mobile station, and the radio channel may receive a transmission signal mixed with an interference signal due to the fading phenomenon. Thus, the signal received in the radio channel is distorted from the original transmission signal so that performance of the mobile communication system is deteriorated. The fading phenomenon may distort amplitude and phase of the signal received over the radio channel, so the fading phenomenon becomes a main factor interfering with the high-speed data communication in the radio channel environment. Various studies and extensive research have been carried out in order to solve the fading phenomenon. In order to transmit high-speed data in the mobile communication system, it is necessary to minimize the loss derived from the characteristics of the mobile communication channel, such as the fading phenomenon, and the interference of the users. In order to prevent an unstable communication caused by the fading phenomenon, various diversity schemes are adopted in the mobile communication system. One such diversity schemes is a space diversity scheme which uses multiple antennas.
A transmission antenna diversity scheme has been suggested in order to effectively solve the fading phenomenon. According to the transmission antenna diversity scheme, a radio channel receives a plurality of transmission signals, which have experienced the fading phenomenon, in order to correct for the distortion of signals caused by the fading phenomenon. The transmission antenna diversity scheme includes a time diversity scheme, a frequency diversity scheme, a multipath diversity scheme, and a space diversity scheme. In order to transmit the high-speed data, the mobile communication system must reduce the fading phenomenon that exerts a negative influence on the performance of the mobile communication system. The fading phenomenon may reduce an amplitude of a signal from a few decibles to tens of decibles. Thus, the diversity scheme is used in order to solve the above fading phenomenon. For instance, a code division multiple access (hereinafter, referred to as “CDMA”) scheme adopts a rake receiver capable of obtaining a diversity function by using a delay spread of a channel. Herein, the rake receiver is a reception diversity type receiver capable of receiving a multi-path signal. However, the reception diversity type rake receiver is disadvantageous in that it cannot obtain a required diversity gain if a channel has a relatively small delay spread.
The time diversity scheme can effectively deal with a burst error occurring in a radio channel environment by using interleaving and coding schemes. Generally, the time diversity scheme is used in a Doppler spread channel. However, according to the above time diversity scheme, a diversity effect is reduced in a low-speed Doppler spread channel. The space diversity scheme is generally used in a channel having a relatively small delay spread. For example, the space diversity scheme is used in an indoor channel and a pedestrian channel, which is a low-speed Doppler spread channel. According to the space diversity scheme, at least two antennas are used for obtaining a diversity gain. If a signal transmitted through one antenna is attenuated due to a fading phenomenon, a signal transmitted through the other antenna is received in the channel, thereby obtaining the diversity gain. Herein, the space diversity scheme is divided into a reception antenna diversity scheme using a plurality of reception antennas, a transmission antenna diversity scheme using a plurality of transmission antennas, and a multiple input multiple output (hereinafter, simply referred to as MIMO) scheme using a plurality of reception antennas and a plurality of transmission antennas.
Hereinafter, an MIMO-adaptive antenna array (hereinafter, simply referred to as “MIMO-AAA”) scheme, which is one of the transmission/reception antenna diversity schemes, will be described.
According to the MIMO-AAA scheme, signals are received through an antenna array including a plurality of reception antennas, and predetermined weight vectors are applied to signal vectors of the received signals in such a manner that the intensity of the desired signals transmitted to a receiver in a particular transmission direction can be maximized, and the intensity of any undesired signals transmitted to the receiver in an improper transmission direction, that is, the intensity of any undesired signals improperly transmitted to the receiver, can be minimized. In addition, the receiver transmits a signal to a transmitter after calculating a transmission weight vector for the signal, so that a beam of a signal transmitted to the receiver from the transmitter can be effectively recreated. That is, according to the above MIMO-AAA scheme, only a required signal is maximally amplified when the signal is received in the receiver, and the signal is radiated toward the receiver with a maximum intensity, so that the speech quality can be improved and service areas can be enlarged.
Although the above MIMO-AAA scheme can be adapted for various mobile communication systems using a frequency division multiple access (hereinafter, simply referred to as “FDMA”) scheme, a time division multiple access (hereinafter, simply referred to as “TDMA”) scheme, or a code division multiple access (hereinafter, simply referred to as “CDMA”) scheme, the MIMO-AAA scheme will be described in relation to a mobile communication system using the CDMA scheme (hereinafter, simply referred to as “CDMA mobile communication system”) for convenience of explanation.
Hereinafter, the elements of a transmitter and a receiver of a CDMA mobile communication system will be described with reference to FIG. 1.
FIG. 1 is a block diagram of a transmitter and a receiver of a general CDMA mobile communication system.
Prior to explaining FIG. 1, it is noted that the following description is made on the assumption that the CDMA mobile communication system adopts an MIMO-AAA scheme. Accordingly, the transmitter and the receiver must have a plurality of transmission antennas and a plurality of reception antennas, respectively. However, according to FIG. 1, the transmitter and the receiver do not have separate transmission antennas and reception antennas, but the same antennas are used for both the transmitter and the receiver through a time division scheme by using a duplexer. In addition, according to FIG. 1, an N-number of antennas is used. Furthermore, the transmitter and the receiver may be a base station or a mobile station.
Hereinafter, the transmitter of the CDMA mobile communication system will be described.
Referring to FIG. 1, the transmitter includes an encoder 101, an interleaver 103, a transmission beam generator 105, a signal processor 107, a plurality of spreaders including a first to Nth spreaders 111, 121, . . . , and 131, and an N-number of radio frequency (hereinafter, simply referred to as “RF”) processors including a first to Nth RF processors 113, 123, . . . , and 133. In addition, a duplexer 140 is commonly used for both the transmitter and the receiver and an N-number of antennas including a first to Nth antennas 141, 143, . . . , and 145 are also commonly used for both the transmitter and the receiver.
Firstly, if data to be transmitted is created, the data is input into the encoder 101. The encoder 101 encodes the data using a predetermined encoding method, and outputs a signal to the interleaver 103. Herein, the encoding method includes a turbo encoding method or a convolutional encoding method. Upon receiving the signal from the encoder 101, the interleaver 103 interleaves the signal through a predetermined interleaving method in order to prevent a burst error, and outputs the signal to the transmission beam generator 105. Herein, the signal output from the interleaver 103 is represented as “z′k”. Then, the signal processor 107 calculates a weight value based on the signal z′k output from the interleaver 103 and outputs the signal to the transmission beam generator 105. Then, the transmission beam generator 105 generates a transmission beam by taking into considering the signal z′k output from the interleaver 103 and the weight value calculated in the signal processor 107, and outputs the transmission beam to the first to Nth spreaders 111, 121, . . . , and 131. That is, the transmission beam generator 105 receives the signal output from the interleaver 103, creates the transmission beam, and transmits the transmission beam to each of the first to Nth spreaders 111, 121, . . . , and 131 in such a manner that the transmission beam can be transmitted through each of the first to Nth antennas 141, 143, . . . , and 145. Herein, a procedure for creating the transmission beam does not directly relate to the present invention, so a detailed description thereof will be omitted.
A set of signals output from the transmission beam generator 105 is represented as “yk′”. That is, yk′ is a set of signals generated from the transmission beam generator 105 and mapped to a kth antenna.
The first spreader 111 receives a signal y1′ outputted from the transmission beam generator 105 and spreads the signal y1′ by using a predetermined spreading code. After that, the first spreader 111 outputs the signal y1′ to the first RF processor 113. Upon receiving the signal from the first spreader 111, the first RF processor 113 performs an RF-treatment process with respect to the signal and outputs the signal to the duplexer 140. Herein, each of the RF processors includes an amplifier, a frequency converter, a filter, and an analog-to-digital converter so as to process the RF signals. In addition, the second spreader 121 receives a signal y2′ output from the transmission beam generator 105 and spreads the signal y2′ by using a predetermined spreading code. After that, the second spreader 121 outputs the signal y2′ to the second RF processor 123. Upon receiving the signal from the second spreader 121, the second RF processor 123 performs an RF-treatment process with respect to the signal and outputs the signal to the duplexer 140. In the same manner, the Nth spreader 131 receives a signal yN′ output from the transmission beam generator 105 and spreads the signal yN′ by using a predetermined spreading code. After that, the Nth spreader 131 outputs the signal yN′ to the Nth RF processor 133. Upon receiving the signal from the Nth spreader 131, the Nth RF processor 133 performs an RF-treatment process with respect to the signal and outputs the signal to the duplexer 140.
The duplexer 140 controls the signal transmission/reception operations by scheduling a transmission point and a receiving point of the signal under the control of a controller (not shown). In addition, the first to Nth antennas 141, 143, . . . , and 145 can be operated as transmission antennas (Tx. ANT) or reception antennas (Rx. ANT) according to the signal transmission/reception operations of the duplexer 140.
Hereinafter, the receiver of the base station of the CDMA mobile communication system will be described.
The receiver includes an N-number of RF processors including a first to Nth RF processors 151, 161, . . . , and 171, an N-number of multipath searchers including a first to Nth multipath searchers 153, 163, . . . , 173 corresponding to the RF processors, an L-number of fingers including a first to Lth fingers 180-1, 180-2, . . . , 180-L for processing signals regarding an L-number of multipaths, which are searched by the multipath searchers, a multipath combiner 191 for combining multipath signals output from the L-number of fingers, a de-interleaver 193, and a decoder 195.
Firstly, the signals transmitted from a plurality of transmitters are received in the N-number of antennas through a multipath fading radio channel. The duplexer 140 outputs the signal received through the first antenna 141 to the first RF processor 151. Upon receiving the signal from the duplexer 140, the first RF processor 151 performs an RF-process with respect to the signal to convert the signal into a baseband digital signal. Then, the first RF processor 151 sends the baseband digital signal to the first multipath searcher 153. Upon receiving the baseband digital signal from the first RF processor 151, the first multipath searcher 153 divides the baseband digital signal into an L-number of multipath components and outputs the multipath components to the first to Lth fingers 180-1, 180-2, . . . , 180-L, respectively. Herein, each of the first to Lth fingers 180-1, 180-2, . . . , 180-L is mapped with each of the L-number of multipaths on a one-t-one basis to process the multipath components. Since the L-number of multipaths must be considered in relation to each signal received through the N-number of reception antennas, the signal process must be carried out with respect to an N×L number of signals. Among the N×L number of signals, signals having the same path are output to the same finger.
In addition, the duplexer 140 outputs the signal received through the second antenna 143 to the second RF processor 161. Upon receiving the signal from the duplexer 140, the second RF processor 161 RF processes the signal to convert the signal into a baseband digital signal. Then, the second RF processor 161 sends the baseband digital signal to the second multipath searcher 163. Upon receiving the baseband digital signal from the second RF processor 161, the second multipath searcher 163 divides the baseband digital signal into an L-number of multipath components and outputs the multipath components to the first to Lth fingers 180-1, 180-2, . . . , 180-L, respectively.
In the same manner, the duplexer 140 outputs the signal received through the Nth antenna 145 to the Nth RF processor 171. Upon receiving the signal from the duplexer 140, the Nth RF processor 171 performs an RF processes to convert the signal into a baseband digital signal. Then, the Nth RF processor 171 sends the baseband digital signal to the Nth multipath searcher 173. Upon receiving the baseband digital signal from the Nth RF processor 171, the Nth multipath searcher 173 divides the baseband digital signal into an L-number of multipath components and outputs the multipath components to the first to Lth fingers 180-1, 180-2, . . . , 180-L, respectively.
In this manner, among the signals received through the N-number of antennas, L multipath signals are input into the same finger. For instance, first multipath signals of the first to Nth antennas 141 to 145 are input into the first finger 180-1, and the Lth multipath signals of the first to Nth antennas 141 to 145 are input into the Lth finger 180-L. In the meantime, the first to Lth fingers 180-1 to 180-L have the same structure and operational property even though different signals are inputted/outputted to/from the first to Lth fingers 180-1 to 180-L. Thus, only the structure and operation of the first finger 180-1 will be described below as an example.
The first finger 180-1 includes an N-number of de-spreaders including first to Nth de-spreaders 181, 182, . . . , and 183 corresponding to the N-number of multipath searchers, a signal processor 184, for receiving signals output from the first to Nth de-spreaders 181 to 183 and calculating weight values thereof so as to create a reception beam, and a reception beam generator 185, for creating the reception beam based on the weight values calculated by the signal processor 184.
Firstly, a first multipath signal output from the first multipath searcher 153 is input into the first de-spreader 181. Upon receiving the first multipath signal, the first de-spreader 181 de-spreads the first multipath signal by using a predetermined de-spreading code and outputs the first multipath signal to the signal processor 184 and the reception beam generator 185. Herein, the de-spreading code is identical to the spreading code used in each transmitter and the de-spreading process is referred to as “time processing”. In addition, the first multipath signal output from the second multipath searcher 163 is input into the second de-spreader 182. Upon receiving the first multipath signal, the second de-spreader 182 de-spreads the first multipath signal by using a predetermined de-spreading code and outputs the first multipath signal to the signal processor 184 and the reception beam generator 185. In the same manner, a first multipath signal output from the Nth multipath searcher 173 is inputted into the Nth de-spreader 183. Upon receiving the first multipath signal, the Nth de-spreader 183 de-spreads the first multipath signal by using a predetermined de-spreading code and outputs the first multipath signal to the signal processor 184 and the reception beam generator 185.
The signal processor 184 receives signals output from each of the first to Nth de-spreaders 181 to 183 and calculates a set of weight values wk for creating the reception beam. Herein, a set of the first multipath signals output from the first to Nth multipath searchers 153 to 173 is defined as “xk”. That is, “xk” represents a set of the first multipath signals received at a kth point through the first to Nth antennas 141 to 145. All of the first multipath signals forming the first multipath signal set “xk” are vector signals. In addition, wk represents a set of weight values to be applied to each of the first multipath signals received at the kth point through the first to Nth antennas 141 to 145. All of the weight values forming the weight value set wk are vector signals.
In addition, a set of de-spread signals of the first multipath signals forming the first multipath signal set “xk” is defined as “yk”. Herein, “yk” represents a set of de-spread signals of the first multipath signals received at a kth point through the first to Nth antennas 141 to 145. All of the de-spread signals forming the de-spread signal set yk are vector signals. For the purpose of convenience of explanation, the term “set” will be omitted below. It is noted that the parameters having an under-score mark represent a set of specific elements.
In addition, since the first to Nth de-spreaders 181 to 183 de-spread the first multipath signals xk by using the predetermined de-spreading code, the power of the desired signal received through the proper transmission direction can be amplified by a process gain as compared with the power of an interference signal.
Meanwhile, as described above, the de-spread signals yk of the first multipath signals xk are input into the signal processor 184. The signal processor 184 calculates the weight values wk based on the de-spread signals yk of the first multipath signals xk and outputs the weight values wk to the reception beam generator 185. That is, the signal processor 184 calculates an N-number of weight values wk applied to the first multipath signals xk output from the first to Nth antennas 141 to 145 by using the de-spread signals yk of the N-number of first multipath signals xk. The reception beam generator 185 receives the de-spread signals yk of the N-number of first multipath signals xk and the N-number of the weight values wk. In addition, the reception beam generator 185 creates the reception beam by using the N-number of the weight values wk. After that, the reception beam generator 185 outputs a signal as an output signal zk of the first finger 180-1 by combining the de-spread signals yk of the N-number of the first multipath signals xk with the weight values wk of the reception beams. Herein, the output signal zk of the first finger 180-1 may be represented as follows in Equation 1.zk=wkHyk  (1)
The above Equation 1 represents a Hermitian operator, that is, a transpose of a conjugate. In addition, zk, which is a set of output signals zk output from the N-number of fingers of the receiver, is finally input into the multipath combiner 191.
Even though only the operation of the first finger 180-1 has been described above as an example, the other fingers may operate in the same manner as the first finger 180-1. Thus, the multipath combiner 191 receives the signals output from the first to Lth fingers, combines the signals with each other through a multipath scheme, and outputs the signals to the de-interleaver 193. The de-interleaver 193 receives the signals output from the multipath combiner 191, de-interleaves the signals through a predetermined de-interleaving method corresponding to the interleaving method used in the transmitter, and outputs the signals to the decoder 195. Upon receiving the signals from the de-interleaver 193, the decoder 195 decodes the signals through a decoding method corresponding to the encoding method used in the transmitter and outputs signals as the final reception data.
The signal processor 184 calculates weight values wk according to a predetermined algorithm in order to minimize a mean square error (hereinafter, simply referred to as “MSE”) of a signal transmitted from a desired transmitter. In addition, the reception beam generator 185 creates a reception beam by using the weight values wk calculated by the signal processor 184. The process for creating the reception beam such that the MSE can be minimized is referred to as “spatial processing”. Of course, the process for creating a transmission beam such that the MSE can be minimized is also referred to as “spatial processing”. Therefore, when the MIMO-AAA scheme is used for the mobile communication system, the time processing and the spatial processing are simultaneously carried out, which referred to as a “spatial-temporal processing”.
Meanwhile, as mentioned above, the signal processor 184 calculates weight values wk capable of maximizing a gain of the MIMO-AAA scheme according to a predetermined algorithm by receiving the multipath signals before the multipath signals have been de-spread and after the multipath signals have been de-spread in each finger. In the same manner, the weight values wk capable of maximizing a gain of the MIMO-AAA scheme are calculated in the transmitter according to a predetermined algorithm. The signal processor 184 and the transmission beam generator 105 are operated to achieve a minimum MSE. Recently, studies and research have been actively carried out regarding an algorithm for calculating the weight values in order to minimize the MSE. According to the algorithm for calculating the weight values for minimizing the MSE, an error is reduced on the basis of a reference signal. If the reference signal does not exist, the algorithm may provide a constant modulus (hereinafter, simply referred to as “CM”) scheme and a decision-directed (hereinafter, simply referred to as “DD”) scheme through a blind manner.
However, the algorithm for minimizing the MSE on the basis of a reference signal is not adaptable if a channel is subject to a fast fading environment. For example, if a channel is subject to a fast fading environment, such as a fast fading channel, or a higher order modulation environment, such as 16 QAM, it is difficult to obtain through the algorithm the MSE having a minimum value required by a system. Even if the minimum MSE can be obtained through the algorithm, the minimum MSE has a relatively large value. If the minimum MSE is determined with a relatively large value, a gain expected when the MIMO-AAA scheme is applied to the mobile communication system may be significantly reduced, so it is not adaptable for a high-speed data communication system. In addition, since both the transmitter and the receiver must calculate the weight values for creating the transmission beam and the reception beam, respectively, a high load may occur when calculating the weight values.